为什么在goroutine的select中有一个默认子句会让它变慢？
参考以下基准测试代码:
func BenchmarkRuneCountNoDefault(b *testing.B) {
b.StopTimer()
var strings []string
numStrings := 10
for n := 0; n < numStrings; n++{
s := RandStringBytesMaskImprSrc(10)
strings = append(strings, s)
}
jobs := make(chan string)
results := make (chan int)
for i := 0; i < runtime.NumCPU(); i++{
go RuneCountNoDefault(jobs, results)
}
b.StartTimer()
for n := 0; n < b.N; n++ {
go func(){
for n := 0; n < numStrings; n++{
<-results
}
return
}()
for n := 0; n < numStrings; n++{
jobs <- strings[n]
}
}
close(jobs)
}
func RuneCountNoDefault(jobs chan string, results chan int){
for{
select{
case j, ok := <-jobs:
if ok{
results <- utf8.RuneCountInString(j)
} else {
return
}
}
}
}
func BenchmarkRuneCountWithDefault(b *testing.B) {
b.StopTimer()
var strings []string
numStrings := 10
for n := 0; n < numStrings; n++{
s := RandStringBytesMaskImprSrc(10)
strings = append(strings, s)
}
jobs := make(chan string)
results := make (chan int)
for i := 0; i < runtime.NumCPU(); i++{
go RuneCountWithDefault(jobs, results)
}
b.StartTimer()
for n := 0; n < b.N; n++ {
go func(){
for n := 0; n < numStrings; n++{
<-results
}
return
}()
for n := 0; n < numStrings; n++{
jobs <- strings[n]
}
}
close(jobs)
}
func RuneCountWithDefault(jobs chan string, results chan int){
for{
select{
case j, ok := <-jobs:
if ok{
results <- utf8.RuneCountInString(j)
} else {
return
}
default: //DIFFERENCE
}
}
}
//https://stackoverflow.com/questions/22892120/how-to-generate-a-random-string-of-a-fixed-length-in-golang
const letterBytes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
const (
letterIdxBits = 6 // 6 bits to represent a letter index
letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
letterIdxMax = 63 / letterIdxBits // # of letter indices fitting in 63 bits
)
var src = rand.NewSource(time.Now().UnixNano())
func RandStringBytesMaskImprSrc(n int) string {
b := make([]byte, n)
// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
if remain == 0 {
cache, remain = src.Int63(), letterIdxMax
}
if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
b[i] = letterBytes[idx]
i--
}
cache >>= letterIdxBits
remain--
}
return string(b)
}
当我对两个函数进行基准测试时,其中一个函数,在其中RuneCountNoDefault没有default子句,select并且RuneCountWithDefault有一个default子句,我得到以下基准:
BenchmarkRuneCountNoDefault-4 200000 8910 ns/op
BenchmarkRuneCountWithDefault-4 5 277798660 ns/op
检查cpuprofile测试生成的,我注意到带有该default子句的函数在以下通道操作中花费了大量时间:
为什么在goroutine中有一个默认子句select会让它变慢？
我使用的围棋版本1.10的windows/amd64
如果一个或多个通信可以继续,则可以通过统一的伪随机选择来选择可以继续的单个通信.否则,如果存在默认情况,则选择该情况.如果没有默认情况,则"select"语句将阻塞,直到至少一个通信可以继续.
修改基准以计算所采取的继续和默认案例的数量:
$ go test default_test.go -bench=.
goos: linux
goarch: amd64
BenchmarkRuneCountNoDefault-4 300000 4108 ns/op
BenchmarkRuneCountWithDefault-4 10 209890782 ns/op
--- BENCH: BenchmarkRuneCountWithDefault-4
default_test.go:90: proceeds 114
default_test.go:91: defaults 128343308
$
虽然其他情况无法继续,但默认情况下在209422470(209890782 - 114*4108)中采用128343308次,每个默认情况下为纳秒或1.63纳秒.如果你做了很多次小事,那就加起来吧.
default_test.go:
package main
import (
"math/rand"
"runtime"
"sync/atomic"
"testing"
"time"
"unicode/utf8"
)
func BenchmarkRuneCountNoDefault(b *testing.B) {
b.StopTimer()
var strings []string
numStrings := 10
for n := 0; n < numStrings; n++ {
s := RandStringBytesMaskImprSrc(10)
strings = append(strings, s)
}
jobs := make(chan string)
results := make(chan int)
for i := 0; i < runtime.NumCPU(); i++ {
go RuneCountNoDefault(jobs, results)
}
b.StartTimer()
for n := 0; n < b.N; n++ {
go func() {
for n := 0; n < numStrings; n++ {
<-results
}
return
}()
for n := 0; n < numStrings; n++ {
jobs <- strings[n]
}
}
close(jobs)
}
func RuneCountNoDefault(jobs chan string, results chan int) {
for {
select {
case j, ok := <-jobs:
if ok {
results <- utf8.RuneCountInString(j)
} else {
return
}
}
}
}
var proceeds ,defaults uint64
func BenchmarkRuneCountWithDefault(b *testing.B) {
b.StopTimer()
var strings []string
numStrings := 10
for n := 0; n < numStrings; n++ {
s := RandStringBytesMaskImprSrc(10)
strings = append(strings, s)
}
jobs := make(chan string)
results := make(chan int)
for i := 0; i < runtime.NumCPU(); i++ {
go RuneCountWithDefault(jobs, results)
}
b.StartTimer()
for n := 0; n < b.N; n++ {
go func() {
for n := 0; n < numStrings; n++ {
<-results
}
return
}()
for n := 0; n < numStrings; n++ {
jobs <- strings[n]
}
}
close(jobs)
b.Log("proceeds", atomic.LoadUint64(&proceeds))
b.Log("defaults", atomic.LoadUint64(&defaults))
}
func RuneCountWithDefault(jobs chan string, results chan int) {
for {
select {
case j, ok := <-jobs:
atomic.AddUint64(&proceeds, 1)
if ok {
results <- utf8.RuneCountInString(j)
} else {
return
}
default: //DIFFERENCE
atomic.AddUint64(&defaults, 1)
}
}
}
//https://stackoverflow.com/questions/22892120/how-to-generate-a-random-string-of-a-fixed-length-in-golang
const letterBytes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
const (
letterIdxBits = 6 // 6 bits to represent a letter index
letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
letterIdxMax = 63 / letterIdxBits // # of letter indices fitting in 63 bits
)
var src = rand.NewSource(time.Now().UnixNano())
func RandStringBytesMaskImprSrc(n int) string {
b := make([]byte, n)
// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
if remain == 0 {
cache, remain = src.Int63(), letterIdxMax
}
if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
b[i] = letterBytes[idx]
i--
}
cache >>= letterIdxBits
remain--
}
return string(b)
}
游乐场:https://play.golang.org/p/DLnAY0hovQG